Optical applications, such as for optical communications, are well known and gaining in popularity due to the many advantages associated with optical signals relative to electrical signals. For optical applications, there are many types of modulation schemes, each offering various advantages and disadvantages.
For example, optical pulse position modulation (PPM) offers a large signal-to-noise (SNR) improvement relative to some modulation schemes. An M-ary PPM coding scheme, as an example, with M equal to sixteen (four bits of information are encoded in the temporal position of an optical pulse) provides a 3 dB savings in optical power compared to differential phase shift keying (DPSK) modulation.
Proposed PPM optical communication systems generally have certain drawbacks. For example, a proposed optical PPM receiver may be intrinsically analog in nature and require high-bandwidth electronic analog-to-digital converters (ADCs) for digital communication applications. The ADCs are generally difficult to design for frequencies of 10 GHz or greater. Furthermore, the ADCs increase system complexity and may reduce operational margins and reliability.
As another example, a proposed PPM receiver may be based on non-linear optical loop mirrors (NOLM), which may require careful balancing and adjustments. The PPM receiver may also require a number of erbium-doped fiber amplifiers (EDFAs) and optical filters with flat dispersion, which adds further to the complexity and design constraints. As a result, there is a need for improved optical techniques, such as for example improved PPM communication techniques.